This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-013236, filed Jan. 21, 2000; No. 2000-015419, filed Jan. 25, 2000; No. 2000-131610, filed Apr. 28, 2000; and No. 2000-400361, filed Dec. 28, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a magnetic resonance imaging apparatus for detecting a magnetic resonance signal of a nuclear spin in an object to be examined and imaging the interior of the object.
A magnetic resonance imaging apparatus (to be referred to as an MRI hereinafter) used as a medical diagnosing apparatus is an apparatus for imaging the interior of an object by detecting a magnetic resonance signal (to be referred to as an MR signal hereinafter) of a nuclear spin in the object. The MRI apparatus can noninvasively image the interior of an object without any radiation exposure, and hence exhibits high clinical utility.
In general, the MRI apparatus has a substantially cylindrical gantry forming an measurement space. In this gantry, a magnetic unit (e.g., superconductive magnet), a gradient field coil, an RF coil, and the like are concentrically arranged. The magnetic unit generates a static field having a very high strength of about several kilogauss to 10 kilogauss (1 tesla) in the measurement space. The gradient field coil generates a linear gradient field superimposed on this static field such that the gradient field changes over time. The RF coil transmits a high-frequency pulse and receives a high-frequency MR signal obtained from the object.
Note that the gradient field coil is comprised of coils of three channels to generate gradient fields in the x-, y-, and z-axis directions. An measurement space is formed in the central portion of these coils, and an object is carried into the space while being laid on the top of a bed. In a static field, a spatial homogeneity of several 10 PPM or less is required. An imaging area in an measurement space requiring this homogeneity often takes a spherical shape having a diameter of about 500 mm.
In imaging operation of obtaining an MR image by the MRI apparatus, the above magnetic unit, gradient field coil, and RF coil are driven in accordance with a desired pulse sequence. More specifically, linear gradient fields in the x-, y-, and z-axis directions are superimposed on an object placed in a static field in accordance with a pulse sequence, and a nuclear spin in the object is magnetically excited by a high-frequency signal having a Larmor frequency. Upon this excitation, an MR signal is generated. This MR signal is detected by the RF coil. By reconstructing the detected MR signal, an MR image of the object is obtained as, for example, a two-dimensional tomographic image.
Recently, there has been a growing need to shorten the time required for imaging in such an MRI apparatus, and a pulse sequence for switching gradient fields of high strength at high speed (inverting polarity at high speed), e.g., a high-speed EPI (Echo Planar Imaging) method, has been put into practice.
When a pulse current flows in the gradient field coil, an electromagnetic force acts on the gradient field coil at the leading edge of a pulse or polarity inversion to make the coil unit mechanically deform owing to the interaction between the electromagnetic force and a static field. As described above, the gradient field coil has coils of three channels which generate gradient fields in the x-, y-, and z-axis directions, and these three gradient field coils are frequently switched at high speed.
The overall coil unit including the gradient field coil and a magnetic vessel for supporting the coil vibrates due to the mechanical distortion of the coil unit. This vibration generates aerial vibration to cause noise. The vibration also generates an impulsive sound. When a gradient field pulse is inverted at high speed, in particular, this vibration increases. As the operation speed increases, therefore, noise increases. The level of this noise becomes 100 dB(A) or more. The noise reverberates in the housing of the gantry in which the object lies or the sealed vessel to produce a larger impulsive sound. This makes the object (patient) feel fear, insecure, and unpleasant.
To prevent an object from being hearing-impaired by this noise, the object is made to wear earplugs or headphones. However, it is troublesome for the patient to wear the earphones or headphones, and they may interfere with imaging operation depending on the imaging position. There have been various proposals for the suppression of the occurrence of such noise. The present applicant has proposed an MRI apparatus designed to prevent air-born or solid-born propagation of noise or vibration by housing a gradient field coil in a sealed vessel with high airtightness as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-256146, U.S. Pat. No. 5,793,210, Jpn. Pat. No. 2642348, and Jpn. Pat. Appln. KOKAI Publication No. 10-118043.
In the conventional MRI apparatus having a silencing mechanism, no measures are taken against a case wherein the silencing function is impaired for some reason. For example, the sealed vessel housing the gradient field coil can prevent propagation of noise caused by the gradient field coil because a vacuum is held in the vessel. If the vacuum in the sealed vessel is lost while an MR image of an object (patient) is taken, large noise is produced abruptly. This makes the object feel fear, insecure, and unpleasant, and may impair the hearing of the object.
The vacuum in this sealed vessel can be maintained by reducing the leak amount to zero. In practice, however, current leads for supplying driving currents to the gradient field coil and a cooling system extend through the sealed vessel, and the vessel has many joint portions. External air flows little by little into the sealed vessel mainly through these portions, and hence the vacuum cannot be maintained for a very long period of time. For this reason, a relatively inexpensive rotary vacuum pump is continuously driven to always exhaust internal air to maintain a vacuum in the sealed vessel.
If, however, the vacuum pump is continuously driven, oil and parts deteriorate quickly. This makes it necessary to frequently perform maintenance. In addition, the service life of the vacuum pump is shortened. Furthermore, the power consumption becomes high, and the running cost becomes high.
It is an object of the present invention to reduce noise in imaging operation.
A magnetic resonance imaging apparatus generates an MR signal from an object by applying a gradient field pulse generated by a gradient field coil and a high-frequency magnetic field pulse generated by a high-frequency coil to the object in a static field, and reconstructs an image on the basis of the MR signal. The gradient field coil is housed in a sealed vessel. The internal air in the sealed vessel is exhausted by the pump to prevent noise. By controlling the operation of the pump using a control circuit, noise in imaging operation can be reduced more effectively.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.